Thermionic valve circuits



May 28, 194:0.

P. F M. GLOESS THERNIONIC VALVE CIRCUITS Filed April 16, 1937 Fig. .1;

Patented May 28 1940 UNITED STATES PATENT? OFFICE.

I THERMIQNIC VALVE CIRCUITS Paul Francois Marie Gloess, Paris, France, as-

Signor to International Standard Electric Corpcration, New York, N. Y. j 1

Application April 16, 1937, Serial'No; 137,304

In France. May 16, 1936 6 Claims. (01.179-471) The present invention relates to improvements in devices such as amplifiers adapted to employ very weak currents, particularly to devices comprising photoelectric cells. The arrangements acording to the invention are particularly ad vantageous in systems such as television systems where a wide band of frequencies is to be amplified.

One object of the invention is to provide ampli 2 fier'devices in which the parasitic noises are considerably reduced or partially'or, totally eliminated.

Another object of the invention is to provide coupling circuits between photoelectric cells and vacuum tubes employed in such amplifiers.

The system considered by way of' example in the following description comprises a simple photoelectric cell element connected to a vacuum tube amplifier, and may be considered as representative of other morecomplicated arrangements such as electron beam devices, for example, theiconcscope associated with amplifiers. In such a system the harmful noises are mainly In the majority of cases, however, the noise due to thermionic agitation in the coupling resistance between the photoelectric cell and the vacuum tube is the most important and it is a particular object of the invention to reducethis noise.

Another object of the invention is to reduce the noise due-to the fluctuation of the grid cur rent which becomesmoretroublesome when the noise of the thermionic agitation in the coupling circuit has been stified. i

Still another object of the invention is to reduce the noise due to the thermionic agitation in the external resistances of the plate .of the tube.

The features of the invention are set out in the appended claims and will be clearly'unden;

stood by means of the following description read in relation to the attached drawing in which:

Fig. 1 shows an example of coupling between aphotoelectric cell and a vacuum tube of an invention;

Fig. 2 shows an example of coupling between a photoelectric cell and a vacuum tube of anamplifier employing a return feed; and

amplifier according to the characteristics of the Fig. '3 shows an example of coupling adapted to reduce distortion in signals due to the grid resistance of the tube.

Fig. 1 shows aphotoelectric cell CP and a vacuum tube TV and their coupling circuit. In thiscoupling circuit, R represents a pure resistance which is the resultant ofthe insulating resistance'of the photoelectric cell, the resistance of the vacuum tube and of an additional resistance. When a circuit is employed for operation on a wide frequency band, the additional resistance normally placed in the circuit is rela tively .low-(for example of the-order of 10,000 to 100,000 ohms) in order to reduce the eifect of the capacity C which tends to reduce the gain for high frequencies. For such values of the photoelectric cell resistance other components coming in the resistance R may be'neglected. The capacity C mentioned and shown in the drawing is a pure capacity and its value is de termine'dby, the capacities offered by the photoelectric cel1,'the additional resistancectheconnection' line and the vacuum tube.

The intensity of the current produced by the. effect of light on the photoelectric'cell will be called I and the difference-of potential at the input of the vacuum tube will m indicated by Es.

-This difference of potential at the terminals of the tube is *equal'tol. Z, if Z represents the impedanceof R and of C in parallel. Consequently:

' E [R I I The difference of potential due to the thernrionic agitation at the input of the tube for a frequency bandequal to 1' is at the input of the tube;

'E =2 /m in'which relation Zc is the Boltzmann constant, T the absolute temperature and r the resistive component of the combination of R and C in parallel. Replacing 1* by its value '1 +w C R the difference of potential ET is Written:

In order to improve the signal-to-noise ratio of a photoelectric cell and an amplifier and supposing that the current I generated by light in the cell has been taken as great as possible and the temperature T as small as possible, the resistance R must thus be increased.

However, the high frequencies will then be considerably attenuated. It is, however, always possible to compensate this attenuation by a corrector network suitably provided.

In the example under consideration the combination of R and C has been considered, but it is clear that any other combination of impedances may be considered without abandoning the characteristic of the invention which consists in reducing the resistive component of the combination of impedances considered.

When the resistance R has no upper limit and is given an extremely high value, the resistance of the photoelectric cell and of the vacuum tube and the grid-cathode resistance are no longer negligible with respect to the resistance shunted on the connecting circuit, and as in general these resistances are not constant, the device tends to become unstable. Even for resistances of 10 to megohms it may sometimes be necessary to avoid variations in the insulation resistances by drying the air surrounding the envelopes of the photoelectric cell and the vacuum tube. Alternatively or in addition the tube may be chosen and operated so that the grid current is of low intensity and consequently negligible.

In this case in which the grid current and the insulation resistance no longer give a. limit to the value of the resistance, the noise introduced by the resistance of the tube and the output plate circuit must then be considered. There is no reason to increase the input resistance further when the noise due to the thermionic agitation therein to the high frequencies is of the same order of intensity as the other noises. In order to take the greatest advantage of the reduction of this noise due to thermionic agitation it is consequently necessary to choose a tube giving only slight noises due to the effects of rapid or wavy fluctuation of the anode current, and to avoid any secondary ionisation and emission. The total noise produced by the tube should not be greater than the unavoidable noise due to the thermionic agitation in the: plate circuit. One method of obtaining this condition is to reduce as far as possible the plate tension, even .if theslope of the tube is thereby reduced.

If the noises have been. reduced so that the noise due to thermionic agitation in the output plate resistance alone becomes important, this resistance must be increased as much as possible so as to increase the amplification and to reduce the noise due to the thermionic agitation due to this resistance. There is no reason to increase this resistance to a great, extent, first because the gain would be little when it became greater than the internal plate resistance, and

secondly because the distortion would be increased. As the output resistance is shunted by the parasitic capacities of the tube, of the coupling and of the connection circuit, the gain at the high frequencies would be less than at the low frequencies.

- This additional distortion can be reduced or wholly compensated by the use of a return feed circuit of which an embodiment is shown in Fig. 2.

The filament-grid circuit of the tube comprising the photo-electric cell also comprises a source E in series with the cell, and a parasitic capacity C in shunt between the connection wires, and a. resistance Rg in series with a battery E" also shunted between. the connection wires. The plate of the tube is connected to the high tension source through a resistance RB. and a parasitic capacity Ca appears in. shunt on the output circuit of the tube. The return feed circuit between the plate and the grid comprises a resistance R: and the parasitic capacity Cag. The values of Rg and Rf are taken so as to have for the value of the return feed 5 a value independent of the frequency:

and a+ f aa+ The amplification ,u of the tube depends on the frequency, but its minimum value is taken such that in the formula of the resultant gain:

u M l #5 u= =constant Another effect of the return feed is to reduce at very low frequencies the incoming load which otherwise would be considerable.

The return. feed must be such that the remaining gain of the tube is still considerable. It

should also be noted that the existence of a return feed slightly reduces in itself the signal; noise ratio.

The distortion due to the presence of the parasitic capacities in parallel with the high resistance of the circuit can also be corrected in another way. When a wide frequency band is to be transmitted it may become possible to ignore the frequencies lower than a certain value (for example, for a television image of 240 lines, the frequencies lower than 5000 p. p. s.). These frequencies may be eliminated by any well known means such as a filter F having a. low frequency cut-off. The attenuation for all the frequencies higher than this value can then follow a simple law and may be compensated in a simple manner by a suitable network of resistances and capacities. In the case inv which the lower frequencies are partially required they can be reintroduced by separate means.

The suppression. of the very low frequencies also has the advantage that the loading of the tube is much smaller because these frequencies iii) would always have a very high amplitude on account of the high value of the grid resistance.

By ignoring the very low frequencies and keeping at a high level the lowest frequencies which must be considered, the following advantages can be obtained:

1. The mechanical noise cannot disturb the precautions from this point of view.

2. The induction due to powerful low frequency fields does not have to be considered, which is advantageous on account of the difficulties met in low frequency systems. 1

3. The current variations due to the feed networks have no influence onthe device. The feed circuits, which before were expensive on account ofthe difiiculty of filtering low frequencies, can thus be simplified.

4. The decoupling between the various parts of the feed systemis rendered easier.

Fig. 3 represents a circuit including means for correcting the distortion due to a high grid re sistance. The photoelectric cell CP is connected to a vacuum tube TV! by a circuit comprising the batteries E and E and by the resistance R; shunted by the parasitic capacity C. This tube, in the output of which is placed in series with the battery E1, the anode resistance Ra. shunted by the parasitic capacity Ca, is provided with a re'-' turn feed obtained by a circuit comprising the resistance Rf shunted by the parasitic capacity Cag.

The connection between the tube TV] and a second tube TV2 is composed of a coupling condenser C a correction circuit comprising in par allel the resistance Rc and the capacity Co and finally in series with the source E1 the grid resistance Rg shunted by the parasitic capacity C' The values Race, R'gc'g, R000 are low and the correction will be almost complete for the band width concerned provided that the time constant Race has the same value as the constant RgC.

In the case in which H is very high the input have only been so described by way .of example and that the invention may be applied to various devices in which the parasitic noises must be reduced. a

What is claimed is:

1. An amplifier comprising a thermionic valve having a plate resistance of such great magnitude that noise due to thermionic agitation in said resistance is small and the, amplifier gain at high frequencies tends to decrease and means for counteracting the decrease of gain comprising a feed-back circuit including a resistance connected between the anode and grid of said valvefor feeding a portionvof the amplifier out put back to the input in phase opposition to the signal to be amplified.

2. An amplifier according to claim 1 in which theratio of the feed-back resistance to the gridleak resistance of the valve approximates to the ratio of the inherent input capacity to the inherent capacitylacross the feed-back resistance.

3. A negative feed-back amplifier in which the feed-back circuit comprises an impedance connected between the anode and control grid of the valve said feed-back circuit having a time constant approximating to that of the input impedance of the valve whereby the ratio of feedback with respect to output substantially independent of frequency. 1

4. An amplifier according to claim 1, further comprising means for suppressing all frequencies below a predetermined frequency range.

5 An amplifier having a large plate resistance to reduce noise due to thermionic agitation, comprising a thermionic valve so constructedthat substantially no grid-current flows, [means for compensating'the high frequency gain losses introducedby said large resistance, comprising a feed-back circuit connected between the anode and grid of said valve for feeding back a, portion of the amplified output back to the input in phase opposition. n v

6. An amplifier according to claim 5, in which the ratio of the feed-back resistance to the gridleak resistance of the valve is substantially equal to the ratio of the inherent. input capacity of the valve to the inherent capacity across the feedback resistance.

PAUL FRANCOIS MARIE GLOESS. 

